The present invention relates to a switching power supply device and a semiconductor integrated circuit employed therein, and to a technique effective if applied to, for example, a switching power supply device mounted to a battery-driven small portable electronic terminal, and a semiconductor integrated circuit used therein.
Electronic apparatuses each equipped with a microprocessor as a system control device have been on the increase in recent years. An operating frequency of the microprocessor (hereinafter called “CPU”) tends to increase more and more. The maximum operating current is also on the increase with the increase in the operating frequency. On the other hand, a portable electronic equipment or the like with the CPU built therein often adopts a system for stepping up or down a battery voltage by means of a switching regulator and supplying an operating current to the CPU. However, when the operation of the CPU is not necessary to lessen the battery exhaustion, the whole CPU or some circuits in the CPU are deactivated. Therefore, the width of a change in current consumption of the CPU tends to increase with the increase in the maximum operating current of the CPU. Thus, one excellent in transient response to a change in output current has been required as a power supply device for supplying an operating current to the CPU.
There has heretofore been known one called “hysteresis current mode control system” as a switching regulator excellent in transient response (see, for example, the following U.S. Pat. No. 5,825,165). The switching regulator of the conventionally proposed hysteresis current mode control system has a current sense resistor for detecting a current that flows through a coil connected in series with a coil, and an error amplifier which outputs a current proportional to an error voltage between a voltage (feedback voltage) obtained by dividing an output voltage by a resistance division circuit and a reference voltage. An error voltage expressed in the product of both the value of a resistor connected between a connecting node of the coil and the sense resistor and an output terminal of the error amplifier, and the output current of the error amplifier is compared with the output voltage by a comparator having hysteresis. When a voltage drop developed across the sense resistor exceeds “error voltage+hysteresis voltage”, a main switch that causes current to flow into the coil is switched from on to off, and a sync switch that acts so as to reduce the current supplied to the coil in synchronism with the main switch is switched from off to on. When the voltage drop developed across the sense resistor falls below the error voltage, the main switch is switched from off to on and the sync switch is switched from on to off, thereby controlling the output voltage so as to be constant.
The switching regulator of such a hysteresis current mode control system effects feedback such that the on time of the main switch is made long when the output current increases, and the on time of the main switch is made short when the output current decreases, thereby to prolong the on time of the sync switch, thereby making it possible to quickly respond to a change in output current and make the output voltage constant.
Patent Document 1: U.S. Pat. No. 5,825,165
However, it turned out that the conventional switching regulator of hysteresis current mode control system had the following problems. Firstly, since it has the sense resistor connected in series with the coil, power wastefully consumed or used up by the sense resistor increases. Further, since the power loss increases as the maximum operating current of the CPU becomes large, this leads to an increasingly reduction in power efficiency from now on. It is considered that the value of the sense resistor is lessened to reduce the power loss. However, since a monitor voltage cannot exceed a hysteresis voltage of a comparator where the value of the sense resistor is excessively reduced, a defective condition occurs that a switching frequency is not fixed and the ripple of the output voltage becomes large.
Secondly, since the output of the error amplifier must follow the change in the output current, the characteristic of response to the change in the output current is delayed by the intervention of the error amplifier. Since the error amplifier generally needs a phase compensating circuit for preventing oscillations, a circuit scale becomes large by the provision of the phase compensating circuit.
Thirdly, when the resistance value of the sense resistor is Rcs, the switching frequency fsw of the regulator is expressed in the following equation (1):fsw=Vout(Vin−Vout)·Rcs/Vin·Vhys·L  (1)
It is understood from the above equation (1) that the switching frequency fsw depends on the inductance L of a coil. Therefore, the switching frequency changes depending on variations in manufacture of the coil, variations in temperature, and a dc current overlap characteristic. An electronic apparatus having a communication function and an audio reproducing function has the fear of occurrence of beat noise in an audio band due to electromagnetic interference. Incidentally, the dc current overlap characteristic means a phenomenon that the inductance of the coil changes according to the magnitude of a dc current that flows through the coil.